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What fraction of students in a class have blue ayes?

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Source: — Data Sufficiency |

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by Jay@ManhattanReview » Mon Dec 11, 2017 11:16 pm
swerve wrote:What fraction of the students in a class have blue eyes?

1) 10% of the girls have blue eyes.
2) 20% of the boys have blue eyes.

The OA is E.

Please, can any expert explain this DS question for me? I have many difficulties to understand why that is the correct answer. Thanks.
Clearly, each statement alone is insufficient. So, let's combine them.

(1) and (2) together

Say the number of boys = b, thus, the number of boys with blue eys = 10% of b = 0.1b; say the number of girls = g, thus, the number of girls with blue eys = 20% of g = 0.2g

The number of students in the class with blue eyes = (01.b + 0.2g)

The fraction of students that have blue eyes = (01.b + 0.2g) / (b + g). We can't get the answer. Insufficient.

The correct answer: E

Hope this helps!

-Jay
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